1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method, and more particularly, to a testing technique of the opening state of a hole, such as a via-hole and a contact hole, which is formed in a process of manufacturing a semiconductor device.
2. Description of the Related Art
Conventionally, a scanning electron microscope (SEM) is used to test a contact hole and a via-hole formed in a process of manufacturing a semiconductor device. As such a technical field, for example, a technique is well known which is described in "Semiconductor World" (issued by Press Journal Corporation, Vol.16, No.9, pages 76-79, July 1997).
In the test method using the SEM, a primary electron beam is scanned while being irradiated to the surface of a semiconductor substrate. Then, secondary electrons generated from the bottom surface of a hole formed in a semiconductor substrate are detected by a secondary electron detector. It is determined based on the detected result by the secondary electron detector whether or not the hole is uniformly opened up to the bottom thereof by etching.
When the hole is perfectly opened, namely, when an insulating film is perfectly removed, the secondary electrons are generated from a lower conductive layer of the bottom of the perfectly opened hole. The generated secondary electrons pass through the hole and then are detected by the secondary electron detector. On the other hand, when an imperfectly opened hole is formed, the secondary electrons are generated from an intermediate insulating film as the bottom of the imperfectly opened hole. The generated secondary electrons pass through the imperfectly opened hole and then are detected by the secondary electron detector. In other words, when the intermediate insulating film is not perfectly removed and remains to cover the lower conductive layer, the secondary electrons generated from the surface of the insulating film at the bottom of the hole pass through the imperfectly opened hole and then are detected by the secondary electron detector.
In this way, the principle of detecting the above-mentioned perfect opening utilizes a fact that an amount of secondary electrons arriving at the secondary electron detector is different between when the perfectly opened hole is formed and when the imperfectly opened hole is formed.
It could be considered that this difference between the amounts of detected secondary electrons depends on a difference between charge accumulating capabilities in the bottom surfaces of the holes rather than a difference between materials of the bottom surfaces of the holes. That is, in the case of the perfectly opened hole, the irradiated primary electron beam arrives at the lower conductive layer and the electrons are grounded through the lower conductive layer. Thus, the electrons are not accumulated. However, in the case of the imperfectly opened hole, the electrons cannot arrive at the lower conductive layer. Hence, the electrons are accumulated on the surface of the intermediate insulating film of the bottom surface of the hole.
Associated with the development of multi-layer technique from the late of 1990's, in a contact hole and a via-hole, the aspect ratio has become higher. At the same time, the hole diameter has become smaller. For these reasons, it is difficult to observe the bottom of the hole by using the scanning electron beam. The opening in the range of an aspect ratio from 2 to 3 can be observed by using the scanning electron beam. However, in a case of an aspect ratio of a value larger than the above-mentioned value, the opening cannot be observed by using the scanning electron beam. Thus, it is difficult to detect a defective opening.
For the purpose of observation of the bottom of a hole having the high aspect ratio, a research result is described in "Observation of Deep Holes using a New Technique" from Japan Society for Promotion of Science, (Application to Charged Particle Beam, 132-th Committee, 121-th Research Conference, Document, pp. 156-161). Also, there are "Cyclotron SEM" from the same Committee, (113-th Research Conference, Document, pp. 155-160), and "Deep Hole Observation Method by Surface Field Control (FCM Method)" from the same Committee, (117-th Research Conference, Document, pp. 165-169). In addition, there is known "SEMSpec" from KLA Corporation which uses an electron beam current approximately 1000 times larger than that of the conventional SEM. Thus, it has become possible to test the open state.
However, if the SEM is used, the measurement range is generally narrow. This results in a problem that an test throughput is low.
In addition, the SEM method has problems described below. That is, the apparatus itself is large in size and expensive. Further, although there is a need of a contrast test under a condition that resist is coated, the test cannot be carried out because the resist is charged up by electrons.
Further, it cannot be determined in the opening test of a via-hole whether or not a barrier layer such as a TiN layer is kept in good state. This is because the contrast due to a difference between materials such as aluminum and TiN is small in an SEM image.
Further, if the bottom of a hole is covered by only an extremely thin natural oxide film, the primary electron passes through the thin oxide film in the SEM method. This leads to the small difference between a perfectly opened hole and an imperfectly opened hole in charge accumulating capability. As a result, it becomes difficult to detect the imperfectly opened hole.
Under such a situation, another contact hole testing method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 7-83841). In the reference (JP-A-Heisei 7-83841), an ultraviolet laser beam generated from an ultraviolet laser beam source 101 is irradiated to a test target 107 using a rotary multi-surface mirror 105 and a scan lens 106. An ultraviolet light reflected from the test target 107 is received by a reflected light optical system composed of the scan lens 106, the rotary multi-surface mirror 105 and a beam splitter 104. The received reflected ultraviolet beam is focused by a lens 108 and a pin-hole 109 such that an image corresponding to a region for a via-hole of the test target 107 is imaged on a photomultiplier 110. As a result, a reflected ultraviolet detection signal is outputted.
Also, another contact hole testing method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 3-161949). In this method, a laser light having an energy lower than a work function of an upper layer is irradiated to the contact hole. Then, it is determined whether or not photoelectrons are detected from the contact hole. As a result, the open state of the contact is tested on the basis of the determination.
However, if this method is applied to the test of many contact holes, a silicon substrate as a lower layer is gradually and positively charged by the emission of the photoelectrons. This causes a problem that the photoelectrons are prevented from being emitted out of the wafer substrate. For this reason, it becomes difficult to test the many contact holes.
Also, there is a case that a lower aluminum wiring layer is insulated from a silicon substrate by another intermediate insulating film. In such a case, if the above-mentioned opening testing method is applied to the via-holes between wiring layers, the wiring layer is easily and positively charged up, because the capacity of the wiring layer is small. For this reason, it is difficult to continuously test the many contact holes.